552 CEPHALOCHORDA. 



cirri develop and the club-shaped gland degenerates. In consequence 

 of the development of tongue-bars in connection with the gill-slits, 

 the number of the latter is doubled. The hepatic caecum forms 

 (Figs. 292, 293, 296). 



III. The larva, which in essential points now closely resembles the 

 adult, has given up pelagic life and buries itself in the sand. The 

 gill-clefts already formed, which at first were arranged metamerically, 

 shift nearer together, and their number is further increased by the 

 addition of paired tertiary clefts (Willey). These tertiary clefts 

 continue to increase in number throughout life. 



It has already been mentioned (p. 551) that the first primary gill- 

 cleft which arose in the ventral median line of the second trunk- 

 segment soon shifts to the right side of the body. In an exactly 

 similar way, new gill-clefts form successively in the body-segments 

 that follow (Figs. 290, 291), these clefts also lying in the ventral 

 median line and shifting later to the right side of the body. The 

 row of primary clefts now lying on the right side is destined later 

 to take tip its final position on the left side. The number of primary 

 clefts which thus arise one after the other varies from twelve to 

 fifteen, and is usually fourteen. They have a strictly metameric 

 arrangement and, according to Hatschek (No. 8), are intersegmental. 

 The gill-clefts thus correspond to the boundaries of the segments. 



Hatschek (No. 8) regards the above-mentioned entoderm-vesicles as the most 

 anterior pair of gill-clefts. These vesicles correspond to the posterior boundary 

 of the first metamere (represented by the cephalic process of the mesoderm 

 which Hatschek regarded as an undeveloped pair of primitive segments). 

 The peribranehial groove (the anterioi ciliated arch, which is homologous 

 with the ciliated arch of the Tunicates) was regarded by him as ihe second 

 pair of gill-clefts. The clefts which were described above as the first true 

 gill-clefts would, according to this interpretation, represent the third pair. 

 This pair is on the right somewhat smaller than the others; the clefts which 

 follow the ninth are also at first smaller than the rest and do not deviate from 

 their median position to the right. 



The median ventral blood-vessel which, in the pharyngeal region, 

 constitutes the branchial alters, turns somewhat to the right in the 

 branchial region and then runs forward above the row of primary 

 gill-clefts (Figs. 292, si). We have already seen (p. -34 7) that the 

 course of this vessel is diverted to the right by the rudiment of the 

 must anterior pair of gill-clefts. The course of this vessel marks the 

 future ventral median line of the pharynx. 



A longitudinal ridge now sunn arises above the branchial artery on 

 the right side of the body (Fig. 292, k) ; tins is composed of con- 



